Single/Dual/Quad, 270MHz, 1mA, SOT23,

Current-Feedback Amplifiers with Shutdown

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15

Detailed Description

The MAX4180–MAX4187 are ultra-low-power current-

feedback amplifiers featuring bandwidths up to

270MHz, 0.1dB gain flatness to 90MHz, and low differ-

ential gain (0.08%) and phase (0.03°) errors. These

amplifiers achieve ultra-high bandwidth-to-power ratios

with low distortion, wide signal swing, and excellent

load-driving capabilities. They are optimized for ±5V

supplies but also operate from a single +5V supply

while consuming only 1mA per amplifier. With ±60mA

output current drive capability, the devices achieve low

distortion even while driving 150

Ω loads.

Wide bandwidth, low power, low differential phase and

gain error, and excellent gain flatness make the

MAX4180–MAX4187 ideal for use in portable video

equipment such as cameras, video switchers, and

other battery-powered applications. Their two-stage

design provides higher gain and lower distortion than

conventional single-stage, current-feedback topolo-

gies. This feature, combined with fast settling time,

makes these devices suitable for buffering high-speed

analog-to-digital converters (ADCs).

The MAX4180/MAX4181/MAX4183/MAX4185 have a

low-power shutdown mode that is activated by driving

the amplifiers’ SHDN input low. Placing them in shut-

down reduces quiescent supply current to 135µA (typ)

and places amplifier outputs in a high-impedance

state. These amplifiers can be used to implement a

high-speed multiplexer by connecting together the out-

puts of multiple amplifiers and controlling the SHDN

inputs to enable one amplifier and disable all the oth-

ers. The disabled amplifiers present very little load

(0.1µA leakage current and 4pF capacitance) to the

active amplifiers’ output. Note that the feedback net-

work impedance of all the disabled amplifiers must be

considered when calculating the total load on the

active amplifier output.

Application Information

Theory of Operation

The MAX4180–MAX4187 are current-feedback ampli-

fiers, and their open-loop transfer function is expressed

as a transimpedance,

behavior of the open-loop transimpedance is similar to

the open-loop gain of a voltage-mode feedback

amplifier. That is, it has a large DC value and decreas-

es at approximately 6dB per octave.

Analyzing the follower with gain, as shown in Figure 1,

yields the following transfer function:

bandwidth is essentially independent of closed-loop

Layout and Power-Supply Bypassing

The MAX4180–MAX4187 have an RF bandwidth and,

consequently, require careful board layout, including

the possible use of constant-impedance microstrip or

stripline techniques.

To realize the full AC performance of these high-speed

amplifiers, pay careful attention to power-supply bypass-

ing and board layout. The PC board should have at least

two layers: a signal and power layer on one side, and a

large, low-impedance ground plane on the other side.

The ground plane should be as free of voids as possible.

With multilayer boards, locate the ground plane on a

layer that incorporates no signal or power traces.

Regardless of whether a constant-impedance board is

used, observe the following guidelines when designing

the board:

• Do not use wire-wrap boards. They are too

inductive.

• Do not use breadboards. They are too capacitive.

• Do not use IC sockets. They increase parasitic

capacitance and inductance.

• Use surface-mount components rather than through-

hole components. They give better high-frequency

performance, have shorter leads, and have lower

parasitic reactances.

• Keep lines as short and as straight as possible.

V

V

OUT

IN

(

/

)

==

+

GR

R

FG

1

+1

+1

MAX4180-MAX4187

Figure 1. Current-Feedback Amplifier